Burning ovens of this type have been known for a long time. It is important for burning ovens, for example for dental ceramics, to be calibrated, since the restoration result that can be achieved depends greatly upon the extent to which the temperature curve started pursuant to the pertinent burning program is in actuality maintained. In practice, an incorrect calibration can very readily lead to a temperature error of 10° C., which is unacceptable.
So-called silver melt probes have been used for a long time to calibrate burn ovens. Silver typically melts exactly at the temperature of 961.3° C. With the manual silver melt probe, a holder is centrally placed in the burning oven for a horizontally arranged silver wire that is connected on one side. The melting of the silver wire leads to the lowering of the horizontal position of the silver wire and is effected precisely at this temperature point. The dental technician that recalibrates the burning oven adjusts the burning oven so that at this temperature point, the reference temperature, the burning oven indicates precisely 961.3° C.
At greater deviations, however, it is necessary to recalibrate by repeating the manual calibration. In addition, although the process is quite simple, it frequently suffers from errors. For example, the silver probe must not be placed upon a honey-combed support, since this would falsify the measurement result. Furthermore, the silver probe must be positioned exactly in the middle of the oven base in order to enable a reliable reference. Finally, the beginning of the melting depends upon the subjective observation of the dental technician. Furthermore, it is also important that the calibration not be undertaken immediately after a burning process or when the burning oven is cold, since in the first instance the starting temperature is too high and in the second instance the starting temperature would be too low.
Due to these drawbacks, it has been known for a long time to automate the calibration of an oven. For example, DE 4218 032 A1 discloses an oven for carrying out thermal analyses, whereby the temperature of the oven support can be measured by thermocouple elements. The connection wires or leads of the thermocouple elements are guided to the outside.
DE-OS 42 06 540 furthermore discloses the use of a reference temperature element. By means of an electronic evaluation circuit, the actual temperature can be detected, and a separate calibration is desired. However, this approach is very complicated and is in particular dependent upon the precision of the evaluation. In practice a correction signal is derived based upon the melting temperature of tin (231° C.), with this correction signal being intended to permanently correct the thermocouple element located there. To this extent, no independent calibration is effected, but rather a continuous readjustment during the measurement.
The precision of the temperature measurement of all of the known calibration processes could be improved. Therefore, it is recommended in various situations that the entire burning oven be recalibrated at the factory, which of course means a corresponding expense for shipment, but also a down time in the dental technician laboratory during the calibration in the factory and the transport time.
A further problem is the deviation of the temperature profile actually realized by the burning oven from the actual temperatures that are to be maintained. In order to compensate for this drawback, it is known from DE 100 08 603 C2 to use two reference temperatures, namely the melting point of gold (1064.67° C.) and of silver (961.3° C.). In so doing, not only can an actual offset, but also a slope of the error curve, be taken into account. However, the use of two melt wires requires a very complicated process, especially since due to the different materials used one must also avoid the occurrence of contaminations.
Contaminations at the temperature detection element that is commonly used for the temperature control of the oven, which element can also be designated as a control thermocouple element, may also lead to temperature deviations. Due to vapor given off by the material being burned, the temperature detection element can become contaminated, resulting in a temperature deviation. This then requires a special, careful recalibration in order to avoid a very expensive replacement of this control thermocouple element.
It is an object of the present invention to provide a burning oven of the aforementioned general type that has a reliable calibrating step that is not susceptible to error should be possible at relatively low expense.